There are alternatives to the current posture which would enable the UK to inflict "significant damage" and deter aggressors

Submarines could potentially be operated at "reduced readiness" when threat levels are lower

A continuous-at-sea presence is the most "resilient" posture and guarantees the quickest response

Land and air-based delivery systems effectively ruled out

An entirely new system, using cruise rather than ballistic missiles, would be more expensive than renewing Trident

The UK's current four-submarine fleet will reach the end of its lifespan in the 2020s and one of the main arguments surrounds how many "successor" submarines - which take 17 years to build - should be commissioned.

Analysis

Jonathan BealeDefence correspondent, BBC News

The Trident Alternatives Review was never going to settle the debate about the future of Britain's nuclear deterrent.

The review asked three key questions. The first two were: Are there credible alternatives to a submarine-based deterrent? Are there credible submarine-based alternatives to the current proposal - such as modifying the Astute submarines?

In both cases the answer appears to be no. Basing nuclear missile silos on land was never really a starter. Too controversial and too easy to target. And the review appears to conclude that modifying the Astute submarines to carry nuclear cruise missiles would be both more expensive and less effective.

The one hope for the Liberal Democrats is in the last question: Are there alternative nuclear postures, such as a non-continuous at-sea deterrent?

For the Conservatives the answer is still no. Philip Hammond says it would be like having a part time deterrent. He wants a like-for-like replacement.

But the Liberal Democrats argue you could save billions of pounds by having two submarines instead of four. There will be clear blue water between the two parties before the next election.

The report suggests four boats would be required to maintain a continuous-at-sea presence and a smaller fleet would risk "multiple unplanned breaks" in 24-hour patrolling and could affect the UK's ability to respond in crises.

It says the UK could still operate a nuclear weapons system with three or even two boats but that would depend on "political confidence" that there was no chance of an unexpected pre-emptive attack and more regular patrols could be reconstituted.

But Mr Hammond told the BBC that nuclear submarines were the "most complex man-made object on earth" and reducing the numbers available would leave the UK extremely "vulnerable".

"Just because we do not perceive an immediate threat today, does not mean there would not be a threat over the 60-year odd time horizon we are looking at," he said.

"The truth is, at the end of the day, we can have continuous-at-sea nuclear deterrents or we can have a part-time deterrent. The part-time deterrent will save us only trivial sums of money."

But Chief Secretary to the Treasury Danny Alexander accused Mr Hammond of trying to "rubbish the report" and said his Conservatives colleagues "were worried about losing the argument".

'Nuclear ladder'

The UK's nuclear policy had "not moved on very much since the end of the Cold War", he said, and the review showed there were "credible alternatives" to the current arrangements.

"We can move on by ending 24-hour patrols when we don't need them and buying fewer submarines," he said.

"That way we can move down the ladder of disarmament as a country without compromising our national security."

Critics have questioned whether the UK can continue to pay for Trident in its current form. The government estimates renewal costs will be between £15bn and £20bn but anti-nuclear campaigners say the figure will be much higher.

Mr Alexander said £4bn would be saved in the medium to long term from moving to three submarines but the Ministry of Defence says the current cost of operating the Trident fleet is about 5% of the annual £34bn defence budget.

'In denial'

Parliament will debate the findings of the report on Wednesday.

Labour MP John Woodcock, who sits on the Commons defence committee, said the review had "fundamentally discredited" the case for alternatives to Trident.

"The government should publish how much taxpayers' money has been wasted on this pointless two-year exercise which has shown beyond doubt that the Liberal Democrats cannot be taken seriously on defence issues," he said.

But the SNP, which has vowed to remove nuclear weapons from Scottish soil if it wins an independence referendum next year, said the review was "not worth the paper it is written on".

"The Westminster establishment seem to have forgotten that Trident is based in Scotland, and neither the people nor parliament of Scotland want it here," said its Westminster leader Angus Robertson.

"This review is in denial, and panders to the vanity of the Westminster system which wants to keep this out-dated, dangerous arsenal of nuclear weapons on the Clyde."